I think that for many of them, it's actually sincere. It's not a cynical strategy to continue failing to actually represent their constituency and get elected anyway - they actually believe that those on the left who oppose them should be ashamed of doing so.

As a general rule, people aren't consciously evil and destructive. Some certainly are, but many (most?) live in a sort of fantasy world in which they've framed their evil such that it's at least justified if not actually good. They rationalize and excuse all of the concessions they make and build up this whole framework in which what they're doing is right.

And then when they look out at other people from within that (warped) framework, it really does appear to them that those who have not chosen to do as they do are wrong for having done so, and thus justifiably shamed.

And they never stop and pull back and try to analyze things from another perspective, since to do so would risk destroying this whole fantasy world they've built - all of their rationalizations and excuses and comforting misconceptions would come crashing down. And they can't allow that. So they just cling to the fantasy, which means, among other things, charging ahead with the (mis)perception that their critics are shamefully wrong.